Marine Science Institute and Department of Ecology, Evolution and Marine Biology, University of California Santa Barbara, Santa Barbara, California, United States of America.
PLoS One. 2011 Mar 31;6(3):e18320. doi: 10.1371/journal.pone.0018320.
Nutrient enrichment of high-elevation freshwater ecosystems by atmospheric deposition is increasing worldwide, and bacteria are a key conduit for the metabolism of organic matter in these oligotrophic environments. We conducted two distinct in situ microcosm experiments in a high-elevation lake (Emerald Lake, Sierra Nevada, California, USA) to evaluate responses in bacterioplankton growth, carbon utilization, and community structure to short-term enrichment by nitrate and phosphate. The first experiment, conducted just following ice-off, employed dark dilution culture to directly assess the impact of nutrients on bacterioplankton growth and consumption of terrigenous dissolved organic matter during snowmelt. The second experiment, conducted in transparent microcosms during autumn overturn, examined how bacterioplankton in unmanipulated microbial communities responded to nutrients concomitant with increasing phytoplankton-derived organic matter. In both experiments, phosphate enrichment (but not nitrate) caused significant increases in bacterioplankton growth, changed particulate organic stoichiometry, and induced shifts in bacterial community composition, including consistent declines in the relative abundance of Actinobacteria. The dark dilution culture showed a significant increase in dissolved organic carbon removal in response to phosphate enrichment. In transparent microcosms nutrient enrichment had no effect on concentrations of chlorophyll, carbon, or the fluorescence characteristics of dissolved organic matter, suggesting that bacterioplankton responses were independent of phytoplankton responses. These results demonstrate that bacterioplankton communities in unproductive high-elevation habitats can rapidly alter their taxonomic composition and metabolism in response to short-term phosphate enrichment. Our results reinforce the key role that phosphorus plays in oligotrophic lake ecosystems, clarify the nature of bacterioplankton nutrient limitation, and emphasize that evaluation of eutrophication in these habitats should incorporate heterotrophic microbial communities and processes.
大气沉降导致高海拔淡水生态系统的营养物质富化在全球范围内不断增加，而细菌是这些贫营养环境中有机质代谢的关键途径。我们在一个高海拔湖泊（美国内华达山脉的翡翠湖）中进行了两次不同的原位微宇宙实验，以评估细菌浮游生物的生长、碳利用和群落结构对硝酸盐和磷酸盐短期富化的反应。第一次实验是在冰融化后立即进行的，采用黑暗稀释培养法直接评估营养物质对细菌浮游生物生长和雪融化期间陆地溶解有机质消耗的影响。第二次实验是在秋季翻转期间在透明微宇宙中进行的，研究了在没有人为干预的微生物群落中，细菌浮游生物如何对与浮游植物衍生的有机物质同时增加的营养物质做出反应。在这两个实验中，磷酸盐富化（而非硝酸盐）导致细菌浮游生物生长显著增加，改变了颗粒有机物质的化学计量，并诱导了细菌群落组成的变化，包括放线菌的相对丰度持续下降。黑暗稀释培养法显示，磷酸盐富化后，溶解有机碳的去除量显著增加。在透明微宇宙中，营养物质富化对叶绿素、碳和溶解有机物质荧光特性的浓度没有影响，这表明细菌浮游生物的反应与浮游植物的反应无关。这些结果表明，在生产力较低的高海拔生境中，细菌浮游生物群落可以迅速改变其分类组成和代谢，以响应短期的磷酸盐富化。我们的结果强调了磷在贫营养湖泊生态系统中的关键作用，阐明了细菌浮游生物的营养限制性质，并强调在这些生境中评价富营养化时应纳入异养微生物群落和过程。
